4 If you aren't alert, you computer will be infected with viruses

5 If we use much pesticide on vegetables, they will become poisonous

9 Because Nam needs to improve his social skills, he attends the activity

11 How about using public buses instead of motorbike 

12 They enjoy protecting the environment unpolluted 

15 Mr Brown was disappointed that his son didn't write to him

Bn tk:

Câu 2: 

1: \(9-x^2=\left(3-x\right)\left(3+x\right)\)

\(x^2-2xy+y^2-16=\left(x-y-4\right)\left(x-y+4\right)\)

Câu 1: \(x\sqrt{\dfrac{y^2}{x}}=\sqrt{\dfrac{y^2}{x^3}}\)